234.00-3 Turbine, Generator & Auxiliaries - Course 234 ... - Canteach

234.00-3
Turbine, Generator & Auxiliaries - Course 234
STEAM VALVE HYDRAULIC CONTROL
The movement of large control valves to regulate the
steam supply to modern steam turbines, requires amplification
of the control signals in both force and displacement to provide
sufficient force to actuate the valves. Whether the
control signal from the governor is mechanical or electrical,
the signal is converted to a hydraulic fluid pressure for
movement of the governor steam valves. The hydraulic relay
and actuator has no competitor as a force amplifier for
governor steam valve actuation. No other form of mechanical
or electrical amplifier can develop the very high force demanded
by the speed at which the governor steam valves, of
modern turbines, must operate.
Control
Relay
Feedback
Valve
Actuator
Governor
Steam
Valve
Steam Valve Actuation
Figure 3.1
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234.00-3
Figure 3.1 shows the basic hydraulic relay and actuator
associated with a governor steam valve. The governor steam
valve is opened by increasing the hydraulic pressure on the
underside of the actuator piston and compressing the spring:
the governor steam valve is shut by draining oil from the underside
of the actuator piston and allowing the spring to expand
and force the valve shut. The control relay is positioned
either mechanically or electrically to supply fluid
to, or drain fluid from, the actuator. Mechanical or electrical
feedback is provided to position the control relay to
neutral and stop the flow of fluid when the governor steam
valve has moved to the desired position.
There are two hydraulic fluids which are used for governor
steam valve actuation:
(a) turbine lubricating oil, and
(b) a phosphate ester, fire resistant fluid.
Turbine lubricating oil has been used for many years as
the hydraulic fluid for governor steam valve actuation. Lubricating
oil is readily available and does not require a separate
pumping system nor a separate purifying system, when
used as a control fluid. In addition, if the lubricating oil
system should fail, the governor steam valves will shut since
it is lubricating oil which is holding them open. Lubricating
oi 1 has several disadvantages , although they were not
serious enough to preclude its use as a control fluid:
1. The control fluid has
lUbrication. That is,
properties as a control
ties as a lUbricant.
to be compatible with bearing
the fluid is chosen not for its
fluid but rather for its proper-
2. The lubricating oil is subject to all manner of contamination
which tends to degrade its purity. These include:
fibres, water, chlorides, dirt, rust and sludge.
The principle disadvantage of using lubricating oil, as
a control fluid, did not become apparent until turbines became
fairly large.
The larger the steam flow handled by turbines, the larger
the governor steam valves had to be and the greater the
force necessary to move the valves at the required speeds.
Since Force = (Pressure) (Area), the larger force couId be
gained in one of two ways:
1. increase the pressure of the control oil, or
2. make the actuators larger.
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234.00-3
The upper limit of control oil pressure is about I, 000
kPa (a). This is determined by the fire hazard involved in
lubricating oil leaking from the system and soaking thermal
insulation in contact with hot stearn pipes. Since it is virtually
impossible to prevent oil leaks in high pressure hydraulic
systems, the only effective way to eliminate this fire
hazard is to keep the lubricating oil under low pressure. In
addition, since the flow through the control oil system is a
very small percentage of the flow to the bearings, the use of
high pressure control oil requires needlessly pressurizing a
large volume of oil intended for the bearings.
The alternative method of developing a large force, that
of making the actuator larger, also has its limitation. As
the actuators get larger, the volume of oi I which must be
moved to effect a response becomes greater and greater. This
"reservoir effect", as it is called, results in significant
time delays between the initiation of a control signal and
the response, as large quantities of oil are moved about the
control oil system. The time between the initiation of signal
and its execution is known as the dead time or time delay
and in large turbine governing systems the combined effects
of mechanical inertia and oil reservoir effect can result in
significant dead time.
The governing system on the 540 MW Pickering NGSA turbine
represents nearly the limit to which lubricating oi 1
control systems can be pushed. Even in this system the exclusive
use of lUbricating oil for control valve actuators
would have resulted in unacceptable dead times. As a result,
while the governor steam valves and emergency stop valves
have hydraulic actuators, the intercept valves and steam release
valves are air operated. This eliminates the long runs
of control oil piping which would be necessary to supply
these valves with control oil. In addition, the control oil
system at Pickering NGSA is equipped with anticipator devices,
to decrease the response time of the control oil system.
While the use of a lUbricating oil control oil system,
at Pickering NGSA has proved entirely acceptable the use of
lubricating oil on larger units has not been possible.
The dead time associated with large turbines using lubricating
oil control oil systems has been virtually eliminated
by use of high pressure, hydraulic systems, using a fire
resistant fluid. The fire resistant fluid used in most electrical-hydraulic
governing systems is a synthetic phosphate
ester hydraulic fluid. The fluid looks like and feels like a
light mineral oil. It has good lubricating properties and
excellent stability. The FRF used by Ontario Hydro has an
excellent combination of chemical and physical properties:
low particle count, low chlorine content, high electrical resistivity
and negligible corrosion of most metals. This
makes it a good fluid for use in the close tolerance valves,
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limit switches and overrides which are used in electricalhydraulic
governing systems. Above all, FRF virtually eliminates
the fire hazard associated with conventional petroleum
oils leaking onto hot steam lines.
Figure 3.2 shows a typical FRF hydraulic power plant.
The pumping system consists of two 100% duty centrifugal
booster pumps which supply fluid at 680 kPa(g) to either of
two 100% duty coolers and hence to either of two 100% duty
positive displacement screw type main pumps which boost the
pressure to 6800 kPa(g). Isolation valves permit maximum
plant availability during on-load maintenance.
All oil, and particularly the synthetic, phosphate
esters used in FRF systems, have a tendency to pick up impurities,
notably grit, sludge, water and chlorides which tend
to accelerate system corrosion and wear. Because of the use
of small, high pressure components, with close tolerances,
the cleanliness of FRF systems is particularly critical.
A 100 micron suction strainer is located in each centrifugal
pump suction line. Full flow three micron absolute
disposable element filters, fitted with differential pressure
switches and internal by-passes, are located upstream and
downstream of each main pump. This arrangement ensures all
the fluid is filtered before entering the main pump and before
entering the main hydraulic system. In addition, the
fluid in the reservoir is continuously purified of moisture
and acid via a fluid treatment plant which includes a vacuum
dehydrator and a fuller's earth filtration plant.
Immersion heaters within the fluid reservoir are used to
warm the fluid during a cold start. A low capacity, low
pressure pump circulates the fluid through the coolers and
returns it to the reservoir. When the fluid temperature has
reached a safe minimum the centrifugal and main pumps may be
started.
By using a separate control hydraulic system it is possible
to maintain a much higher standard of fluid control, regarding
temperature, viscosity and purity. It is not subject
to variation in temperature to suit bearing requirements or
seal oil requirements, and it is not subject to metallic pick
up from bearings or moisture pick up from steam glands. However,
the need for system cleanliness and hydraulic fluid
purity, in high pressure, FRF governing systems is considerably
more critical. Removal. of impurities which could foul
and eventually score the electrical-hydraulic control valves
is essential in any high pressure hydraulic system. Control
valve clearances are extremely small and the valves are particularly
susceptible to sticking, scoring and eventual
internal leakage. In addition, electrically actuated valves
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U1
Heater
Reservoir
Preheat
Pump
Fluid
Treatment
Plant
Cooler
FRF Power Plant
Figure 3.2
Main
Pump
To Hydraulic
Control System
N
W
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234.00-3
generally have little reserve power to free galled or sticking
stems. The requirement for periodic testing of an electrical-hydraulic
governing system, to insure freedom of valve
movement, is doubly beneficial, in that fluid flow through
the control valves keeps the valves clean. There is a great
amount of practical experience which indicates that if a
hydraulic control valve is not exercised for several months
it will probably not operate.
FRF is reasonably non-toxic to the skin and exposure
through soiled clothing presents a minimal hazard although
FRF entering the eyes can cause a burning sensation and cause
subsequent irritation. Phosphate esters can cause fatal poisoning,
however, if inhaled in large quantities or if ingested
in even moderate amounts. Under usual station operating
conditions, inhalation of the vapor is almost impossible due
to the low volatility of the fluid. From a personal safety
standpoint, FRF can be harmful and special care should be
taken to prevent ingestion, inhalation or absorption through
the skin by persons who handle it. However, it can be handled
safely if certain precautions are taken. These include
no smoking or eating while handling the fluid, use of rubber
gloves and safety glasses and availability of an eye wash
fountain.
From an environmental standpoint, FRF can present a
potential problem due to its toxicity and relative stabili
ty. For this reason disposal procedures for this fluid
should be carefully adhered to.
ASSIGNMENT
1. Define "dead time" and "reservoir effect".
2. Explain why there is a limit to the size to which turbines
with lubricating oil fluid control systems can be
built.
3. Why is hydraulic oil used for governor steam valve actuation?
4. What are the advantages of an FRF fluid control system
over a lubricating oil control system?
5. What are the problems associated with an FRF control
system?
6. Discuss the need for fluid purity and cleanliness in an
FRF control system.
R.O. Schuelke
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